1. Field of the Invention
The present invention relates to a method for forming a photoelectric composite board which has an optical circuit and an electric circuit, and especially relates to a method for forming the photoelectric composite board by which a photoelectric transducer can be mounted on a photoelectric composite board simply in a high precision.
2. Description of the Related Art
According to rapid progress of wider bandwidth of a communication infrastructure and significant improvement of data processing capacity of a microcomputer and so on, it is needed to provide a data processing circuit having a very fast data transmission speed. Under such a circumstance, data communication by optical signals is considered as a means to exceed a limit of data transmission speed by electric signals, so that a photoelectric composite board, in which an optical circuit is mounted on an electric circuit such as a printed circuit board, is developed. An optical coupler such as a light deflector or grating is formed on the photoelectric composite board, and a photoelectric transducer such as a laser diode or a photodiode is mounted on electrode pads on the photoelectric composite board. Signals are transmitted between the optical circuit and the electric circuit through the optical coupler.
The electrode pads on which the photoelectric transducer is mounted are generally formed by the same method as that to form electrode pads of the printed circuit board to which electronic elements are mounted. More specifically, in a conventional method for forming the photoelectric composite board, electric circuits and electrode pads are formed on a surface of the circuit board with using normal photoresist processes, and a photo-mask is aligned on the circuit board on the basis of a reference point provided on the circuit board. Subsequently, solder resist layers such as UV-hardening resin films are formed on outer peripheries of mounting lands corresponding to the photo-mask, so that electrode pads on which the photoelectric transducer is mounted, consequently.
However, when the photoelectric transducer is mounted on the electrode pads formed by the above mentioned conventional process, center of light emitting portion or a light receiving portion of the photoelectric transducer to be mounted is displaced due to contraction of resin or contraction of the circuit board made of a resin generated in a heating process for forming a light guide or displacement of the photo-mask in alignment thereof. Thus, it is difficult to mount the photoelectric transducer at a position accurately so that light can enter into or exit from the light guide effectively.
JP 2005-17394 A discloses a method to mount a photoelectric transducer on a circuit board accurately by forming circuits through processes of aligning a mask-patter on the basis of a positional relation between a light deflector and a positioning mark previously formed on a circuit board and forming solder resist layers. The circuits, however, are formed with using the photo-mask, so that the displacement of the photo-mask itself occurs, and thus, it is difficult to form the electrode pads for photoelectric transducer accurately. Consequently, a disagreement of several tens μ m between optical axes is generated when light emitted from the photoelectric transducer enters into a light deflector or light exit from the light deflector enters into the photoelectric transducer. A large amount of leakage loss of signals in transform between an optical circuit and an electric circuit occurs.